For goals like a higher speed of operation and a saving of power consumption of microelectronic devices, the challenge to higher integration of large-scale integrated circuits continues. To meet increasing demands for shrinkage of circuit patterns, the advanced semiconductor microprocessing technology becomes important. For example, the technology for shrinkage of circuit-constructing wiring patterns and the technology for shrinkage of contact hole patterns for cell-constructing inter-layer connections become essential.
The advanced microprocessing technology relies on the photolithography using photomasks. The photomask is one important area of the miniaturization technology as are the lithography system and resist material. For the purpose of obtaining a photomask having a fine-size wiring pattern or fine-size contact hole pattern as mentioned above, efforts are made to develop the technique of forming a more fine and accurate pattern on a photomask.
Since the photolithography for microprocessing semiconductor substrates employs reduction projection, the size of a pattern formed on a photomask is about 4 times the size of a pattern formed on a semiconductor substrate. This does not mean that the accuracy of the pattern formed on the photomask is accordingly loosened. It is necessary that the photomask pattern be formed at a high accuracy.
At the present, the size of a circuit pattern written on a semiconductor substrate by photolithography is far smaller than the wavelength of exposure light. If reduction exposure is carried out using a photomask having a pattern which is a mere 4-time magnification of the circuit pattern, the photomask pattern is not faithfully transferred to the resist film due to interference of exposure light and other impacts.
Super-resolution masks addressing the problem include OPC masks in which the so-called optical proximity correction (OPC), i.e., the technology for correcting the optical proximity effect of degrading transfer properties is applied to photomasks and phase shift masks which cause a phase shift of 180° between adjacent pattern features to establish a sharp intensity distribution of incident light. For example, in some OPC masks, an OPC pattern (hammer head, assist bar or the like) having a size of less than half of a circuit pattern is formed. The phase shift masks include halftone, Levenson and chromeless types.
To form a photomask pattern on a photomask substrate at a high accuracy, a resist film formed on the photomask blank must be patterned at a high accuracy. In general, a photomask pattern is formed by starting with a photomask blank having a light-shielding film on a transparent substrate, forming a photoresist film on the photomask blank, exposing the photoresist film to light or electron beam (EB) to write a pattern, and developing the photoresist film to form a photoresist pattern. Then, with the photoresist pattern made mask, the light-shielding film is etched or patterned to form the photomask pattern. To obtain a fine photomask pattern, it is effective to reduce the thickness of a photoresist film (i.e., thinner resist film) for the following reason.
If only a resist pattern is shrunk without reducing the thickness of a resist film, the resist pattern feature functioning as the etching mask for the light-shielding film has a higher aspect ratio (ratio of resist film thickness to feature width). In general, as the aspect ratio of resist pattern features becomes higher, the pattern profile is more likely to degrade. Then the accuracy of pattern transfer to the light-shielding film via the resist pattern as the etch mask is reduced. In extreme cases, the resist pattern partially collapses or strips off, resulting in pattern dropouts. In association with the shrinkage of a photomask pattern, It is necessary that the resist film used as the etching mask during patterning of a light-shielding film be thinned to prevent the aspect ratio from becoming too high. An aspect ratio of up to 3 is generally recommended. To form a resist pattern having a feature width of 70 nm, for example, a resist film thickness of up to 210 nm is preferable.
For the light-shielding film which is etched using the pattern of photoresist as an etch mask, on the other hand, a number of materials have been proposed. In particular, neat chromium films and chromium compound films containing chromium and at least one of nitrogen, oxygen and carbon are generally used as the light-shielding film material. For example, Patent Documents 1 to 3 disclose photomask blanks wherein chromium compound films are formed as the light-shielding film having light shielding properties necessary for the photomask blank for use in ArF excimer laser lithography.
The light-shielding film in the form of chromium compound film is generally patterned by oxygen-containing chlorine dry etching, during which an organic film, typically photoresist film can be frequently etched to a noticeable extent. If the light-shielding film in the form of chromium compound film is etched with a relatively thin resist film made mask, the resist film is damaged during the etching so that the resist pattern is deformed. It is then difficult to transfer the resist pattern accurately to the light-shielding film.
The attempt to endow the photoresist or organic film with high resolution and high patterning accuracy as well as etch resistance encounters a technical barrier. The photoresist film must be reduced in thickness for the goal of high resolution whereas thinning of the photoresist film must be limited for the purpose of ensuring etch resistance during etching of the light-shielding film. As a result, there is a tradeoff relationship between high resolution/patterning accuracy and etch resistance. To mitigate the load to the photoresist to enable film thickness reduction for eventually forming a photomask pattern of higher accuracy, the construction (including thickness and composition) of a light-shielding film to be patterned must be ameriolated.
As to light-shielding film materials, a number of studies have been made. For example, Patent Document 4 discloses a metal film as the light-shielding film for ArF excimer laser lithography. Specifically, tantalum is used as the light-shielding film and tantalum oxide used as the antireflective film. To mitigate the load applied to the photoresist during etching of these two layers, the layers are etched with a fluorine-base gas plasma which causes relatively few damages to the photoresist. Despite a choice of such etching conditions, when two layers, light-shielding film and antireflective film are etched using only the photoresist as etch mask, the mitigation of the load to the photoresist is limited. It is difficult to fully meet the requirement to form a fine size photomask pattern at a high accuracy.
As discussed above, the prior art photomask blank structure is difficult to fully meet the requirement to form a fine size photomask pattern on the light-shielding film at a high accuracy. The problem becomes more serious with the photolithography using exposure light of shorter wavelength and requiring higher resolution, typically light with a wavelength of up to 200 nm (ArF excimer laser of 193 nm, F2 laser of 157 nm).
As the light-shielding film exhibiting a high etch rate during chlorine-base dry etching that enables to mitigate the load to the photoresist for eventually forming a fine size photomask pattern at high accuracy, Patent Document 5 describes a light-shielding film based on chromium and having light elements O and N added thereto. The light element-containing chromium film reduces its conductivity with the increasing content of light elements.
For the fabrication of photomasks, on the other hand, the exposure method using electron beam (EB) is the mainstream of resist patterning. For EB emission, a high accelerating voltage of 50 keV is employed in order to enable further miniaturization. While there is a tendency that the resist reduces its sensitivity in order to achieve a higher resolution, the current density of EB In the EB lithography system experiences a remarkable leap from 40 A/cm2 to 800 A/cm2 from the aspect of productivity enhancement.
When EB is directed to an electrically floating photomask blank, electrons accumulate on the surface of the photomask blank to charge it at a negative potential. An electric field due to the electric charge causes the EB trajectory to be bent, resulting in a low accuracy of writing position. To avoid such fault, the EB lithography system adapted for high energy/high density EB writing is designed such that EB writing is performed with the photomask blank being grounded. For example, Patent Document 6 discloses an earth mechanism for grounding a photomask blank using an earth pin.
However, if ground resistance is significant, the potential of the photomask blank surface increases by the product of ground current and ground resistance, and the accuracy of writing position is accordingly reduced. Also, in examples where no sufficient ground is established, or the photomask blank is not conductive, the ground resistance is very high or infinitely high. If EB writing is performed in this state, an abnormal discharge or substrate failure can occur within the imaging vacuum chamber, causing contamination to the system. Thus the EB lithography system is equipped with a mechanism for measuring a ground resistance prior to the writing step. The threshold of ground resistance is set at 1.5×105Ω, for example. When the ground resistance measured exceeds the threshold, the writing step is interrupted before its start.
For a portion of the system which comes in contact with the photomask blank for grounding, there is the problem that particles are generated when the pin is penetrated through the EB resist film. To overcome the problem, several proposals are made. In Patent Document 6, for example, a cover is shaped so as to surround the earth pin for preventing particles from scattering. After penetration of the pin, a grounding mark is left in the EB resist film. In general, as the grounding mark is smaller, particle generation is more suppressed. One exemplary improvement is to change the shape of the grounding member from the blade type adapted to establish a ground by line contact to the pin type adapted to establish a ground by point contact. Another improvement is made to suppress any enlargement of the grounding mark by a positional shift after the contact of the earth pin.